伊藤 洋一

Context. To reveal details of the internal structure, the relationship between chromospheric activity and the Rossby number has been extensively examined for main-sequence stars. For active pre-main-sequence (PMS) stars, it is suggested that the level of activity be assessed using optically thin emission lines, such as Mg I. Aims. We aim to detect Mg I chromospheric emission lines from PMS stars and to determine whether the chromosphere is activated by the dynamo process or by mass accretion from protoplanetary disks. Methods. We analyzed high-resolution optical spectra of 64 PMS stars obtained with the Very Large Telescope (VLT)/X-shooter and UVES and examined the infrared Ca II (8542 Å) and Mg I (8807 Å) emission lines. To detect the weak chromospheric emission lines, we determined the atmospheric parameters (T_eff and log 𝑔) and the degree of veiling of the PMS stars by comparing the observed spectra with photospheric model spectra. Results. After subtracting the photospheric model spectrum from the PMS spectrum, we detected Ca II and Mg I as emission lines. The strengths of the Mg I emission lines in PMS stars with no veiling are comparable to those in zero-age main-sequence (ZAMS) stars if both types of stars have similar Rossby numbers. The Mg I emission lines in these PMS stars are thought to be formed by a dynamo process similar to that in ZAMS stars. In contrast, the Mg I emission lines in PMS stars with veiling are stronger than those in ZAMS stars. These objects are believed to have protoplanetary disks, where mass accretion generates shocks near the photosphere, heating the chromosphere. Conclusions. The chromosphere of PMS stars is activated not only by the dynamo process but also by mass accretion.

Abstract In this study, we revisit 32 planetary systems around evolved stars observed within the framework of the Okayama Planet Search Program (OPSP) and its collaborative framework of the East Asian Planet Search Network to search for additional companions and investigate the properties of stars and giant planets in multiple-planet systems. With our latest radial velocities obtained from Okayama Astrophysical Observatory, we confirm an additional giant planet in the wide orbit of the 75 Cet system ($P_{\rm {c } } = 2051.62_{-40.47}^{+45.98}\ \rm {d}$, $M_{\rm {c } }\sin i=0.912_{-0.090}^{+0.088}\,\,M_{\rm {J } }$, and $a_{\rm {c } }=3.929_{-0.058}^{+0.052}\ \rm {au}$), along with five stars exhibiting long-term radial velocity accelerations, which indicates massive companions in the wide orbits. We also find that the radial velocity variations of several planet-harboring stars may indicate additional planet candidates, stellar activities, or other understudied sources. These stars include ϵ Tau, 11 Com, 24 Boo, 41 Lyn, 14 And, HD 32518, and ω Ser. We further constrain the orbital configuration of the HD 5608, HD 14067, HD 120084, and HD 175679 systems by combining radial velocities with astrometry, as their host central stars exhibit significant astrometric accelerations. For other systems, we simply refine their orbital parameters. Moreover, our study indicates that the OPSP planet-harboring stars are more metal poor compared to the currently known planet-harboring stars, and this is likely due to the B − V color upper limit at 1.0 for star selection in the beginning of the survey. Finally, by investigating the less massive giant planets (<5 MJ) around currently known planet-harboring evolved stars, we find that metallicity positively correlates with the multiplicity and total planet mass of the system, which can be evidence for the core-accretion planet formation model.